It is known to compression mold hard (i.e., permanent) magnets, as well as soft magnetic cores for electromagnetic devices (e.g., transformers, inductors, motors, generators, relays, etc.) from a plurality of ferromagnetic particles each encapsulated in a thermoplastic or thermosetting polymeric shell.
Soft magnetic cores are molded from polymer-encapsulated ferromagnetic particles (i.e., less than about 1000 microns) such as iron, and certain silicon, aluminum, nickel, cobalt, etc., alloys thereof (hereafter generally referred to as iron), and serve to concentrate the magnetic flux induced therein from an external source (e.g., current flowing through an electrical coil wrapped thereabout). Unlike hard magnets, such soft magnetic cores, once magnetized, are very easily demagnetized, i.e., require only a slight coercive force (i.e., less than about 200 Oersteds) to remove the resultant magnetism. Ward et al. U.S. Pat. No. 5,211,896, for example, discloses one such soft magnetic core forming material wherein the polymeric shell comprises a thermoplastic polyetherimide, polyamideimide or polyethersulfone which, following molding, fuses together to (1) form a polymer matrix embedding the iron particles, and (2) so electrically insulate each iron particle from the next as to significantly reduce eddy current losses, and hence total core losses (i.e., eddy current and hysteresis losses), in AC applications of the cores molded therefrom. Other possible matrix-forming thermoplastic polymers for this and other purposes include the polysulfones, polycarbonates, polyphenylene ethers, polyphenylene oxides, polyacrylic acids, polyvinylpyrrolidone, and polystyrene maleic anhydride among others.
Permanent (i.e., hard) magnets are also known to be compression molded from such ferromagnetic particles as magnetic ferrites, rare-earth metal alloys (e.g., Sm-Co, Fe-Nd-B, etc.), and the like, and are subsequently permanently magnetized. Shain et al. U.S. Pat. No. 5,272,008, for example, discloses one such hard magnet-forming material comprising iron-neodymium-boron alloy particles encapsulated in a composite polymeric shell comprising a thermosetting, matrix-forming, epoxy underlayer overcoated with a thermoplastic polystyrene outer layer. The polystyrene keeps the epoxy coated particles from sticking together before the epoxy is cured.
In Ward et al. U.S. Pat. No. 5,211,896 and Shain et al. U.S. Pat. No. 5,272,008, the shell-forming polymers are dissolved in an appropriate solvent, and mixed with a fluidized stream of the ferromagnetic particles by spray-coating the particles with the solution, using the so-called "Wurster" process. Wurster-type spray-coating equipment comprises a cylindrical outer vessel having a perforated floor through which a heated gas passes upwardly to heat and fluidize a batch of ferromagnetic particles therein. A concentric, open-ended, inner cylinder is suspended above the center of the perforated floor of the outer vessel. A spray nozzle centered beneath the inner cylinder sprays a solution of the shell-forming polymer, dissolved in a solvent, upwardly into the inner cylinder (i.e., the coating zone) as the fluidized ferromagnetic particles pass upwardly through the spray in the inner cylinder. The particles circulate upwardly through the center of the inner cylinder and downwardly between the inner and outer cylinders. The gas (e.g., air) that fluidizes the metal particles also serves to vaporize the solvent causing the dissolved, shell-forming polymer to deposit as a film onto each particle's surface. After repeated passes through the coating zone in the inner cylinder, a sufficient thickness of polymer accumulates over the entire surface of each particle as to completely encapsulate such particle. Ferromagnetic particles have also been coated with polymers by simply mixing the particles in a suitable vessel with the coating polymer dissolved in a suitable solvent, and then volatizing the solvent to dry the particles and leave the polymer adhering the surfaces thereof.
Lubricants have heretofore been added to polymer-encapsulated ferromagnetic particles. Rutz et al. U.S. Pat. No. 5,198,137, for example, mechanically blends or mixes boron nitride lubricant particles with polymer encapsulated particles prior to molding the particles into finished products to improve the flowability of the powder and the magnetic permeability of moldings made therefrom, as well as to reduce the stripping and sliding die ejection pressures. Moreover, certain lubricous stearates, such as ethylene bisstearateamide lubricant particles--sold commercially under the trade name ACRAWAX.TM.), have heretofore been dry mixed/blended with polymer-encapsulated metal particles to improve processability of the particles.
Moreover, my earlier invention, copending U.S. patent application Ser. No. 08/357,890 filed in the names of D. Gay and myself on Dec. 16, 1994 and assigned to the assignee of the present invention provides a mass of ferromagnetic particles each of which is encapsulated in a lubricous polymeric shell comprising a plurality of organic, lubricant particles essentially buried in a film of a soluble thermoplastic binder on the surface of each of the polymer-encapsulated ferromagnetic particles. As these lubricant particles are bonded to the surfaces of the ferromagnetic particles, they are not susceptible to subsequent segregation, and significantly improve (1) the dry particle flowability and hot compactability (i.e., densification) of the encapsulated particles, and (2) the electrical resistivity of moldings made therefrom. High resistivity and high density moldings make the best soft magnetic cores for high frequency AC applications as they provide both high magnetic permeability (attributable to higher density) and low core losses (attributable to good interparticle insulation).
While my prior invention. (i.e., U.S. Ser. No. 08/357,890 supra) significantly improved the properties of polymer-encapsulated ferromagnetic particles and moldings made therefrom, the full effectiveness of the organic lubricant particles used therein is impaired somewhat by the binder which anchors the lubricant particles to the ferromagnetic particles. In this regard the binder, for the most part, either buries or so coats the lubricant particles that their full potential as lubricants is not realized.